With the spread of the Internet, communication has become a part of community infrastructure. The amount of data per user increases year after year, and network traffic also continues to increase. As an optical fiber transmission channel that takes charge of backbone communication among network infrastructures, an optical transmission system having a capacity per channel of 100 Gb/s is put into practice use. Then, in the future, an increase in the capacity of a transmission channel based on a system of 400 Gb/s or a system of 1 Tb/s is required.
There is multi-valuing of a signal, as one of techniques for realizing an increase in the capacity of a transmission channel. Among multi-value modulation techniques, a 16-quadrature amplitude modulation (QAM) signal has already been put into practice use. Practice use of an ultrahigh multi-value modulation signal such as a 32-QAM signal or a 64-QAM signal is essential in order to realize 1 Tb/s.
A QAM signal on which multi-value modulation is performed in a transmitter is received and demodulated in a digital coherent receiver. The digital coherent receiver includes a local light source and a carrier recovery unit. The carrier recovery unit estimates and compensates for a frequency error occurring when the received QAM signal and continuous wave (CW) light, which is output from the local light source, interfere with each other at an optical front end. There are a feed forward type and a decision feedback type in a system in which the frequency error is estimated. An example of a frequency error estimation system of the feed forward type is disclosed in Non-Patent Document 1. However, in this system, a corresponding modulation system is limited to QPSK. Complicated signal processing is required for application to a higher-order modulation system. Consequently, in a multi-value modulation signal equal to or more than 16-QAM, the decision feedback type is often used.